ls: fix crash with --context

ls: fix crash with --context

* src/ls.c (main): Flag that we need to stat()
if we're going to get security context (call file_has_aclinfo_cache).
(file_has_aclinfo_cache): Be defensive and only lookup the device
for the file if the stat has been performed.
(has_capability_cache): Likewise.
* tests/ls/selinux-segfault.sh: Add a test case.
* NEWS: Mention the bug fix.
Reported by Bruno Haible.